1. Field of the Invention
The present invention relates to protective films of: wear resistant components such as tools and dies; machine components and frictional components intended for industrial or home use represented by automobile parts; and magnetic heads of automatic readers for cards or tickets, printers. More particularly, it relates to a coating film and a coated machine component that have an excellent wear resistance and high frictional performance.
2. Description of Related Art
A hard carbon film is generally referred to as a diamond-like carbon (below, may be referred to as DLC) film. Although various designations such as hard non-crystal carbon, amorphous carbon, hard amorphous carbon, i-carbon, and diamond-like carbon are used for DLC, there is no definite distinction between these terms. The nature of the DLC having such various designations lies in being a mixture of diamond and graphite in terms of structure, i.e., having an intermediate structure therebetween. As with diamond, the DLC film is excellent in hardness, wear resistance, solid lubricity, thermal conductivity, chemical stability, and the like. Accordingly, it is being put to use as a coating film of each of various components such as sliding components, dies, cutting tools, wear-resistant machine parts, abrasive components, and magnetic and optical parts.
As the characteristics of the DLC film, mention may be made of the small friction coefficient (below, may be referred to as xcexc) in contact with various counterpart materials including metals such as iron and aluminum, and ceramics such as glass. However, it has been known that the friction coefficient of the DLC film largely varies according to the measurement environment and counterpart materials. In general, for example, with iron-based counterpart materials, it is from 0.15 to 0.4 in an atmosphere, and it is 0.1 or less in a vacuum or in a dry nitrogen atmosphere.
A large number of studies have been done on the xcexc reducing mechanism of the DLC film. In general, it is considered that carbon atoms are attached to the counterpart material from the DLC film, so that the film is graphitized, and undergoes sliding deformation at the C-plane (xcfx80 bond plane) of the graphite to act as a self-lubricating material, resulting in a lower xcexc.
When the DLC film is put to practical use as a hard coating film, it is essential that it has sufficient properties for friction coefficient as low as about 0.1 with respect to iron-based counterpart materials, thin-film hardness influencing the wear resistance, adhesion with the substrate affecting the reliability of the coating. A large number of proposals have been made concerning these conditions.
As particularly predominant means, mention may be made of addition of alloy elements to DLC, and attainment of laminated structure of the coating film. With the addition of alloy elements, it has been reported that, for example, when Si is added, xcexc is from 0.1 to 0.15, but the hardness is improved to about 30 GPa.
Laminated structure of DLC has been recognized as effective means for reduction of internal stress, improvement of adhesion, improvement of durability by an increase in film thickness, improvement of corrosion resistance, and control of electric resistance, and for example, the following technologies have been known:
(1) J-P-A No. 65625/1993 describes a laminated product which has a substrate, a hard carbon film, and a buffer layer made of one or more selected from silicon, germanium, silicon carbide, silicon nitride, silicon dioxide, glass, and alumina, the hard carbon film and the buffer layer being alternately laminated on the substrate, and the outermost layer being the hard carbon film.
(2) J-P-A No.237827/1998 describes a laminated product in which a hard carbon film, or a hard carbon film doped with at least one or more metallic elements, and at least one or more metals, metal carbide, metal nitride, or metal carbonitride are stacked repeatedly and alternately at a cycle of from 1 nm to 3 nm, or a laminated product in which at least two or more hard carbon films doped with different types of metallic elements or different amounts of the same metallic element are stacked repeatedly and alternately at a cycle of from 1 nm to 3 nm.
(3) J-P-A No. 226874/1998 describes a laminated product having carbon, or carbon and hydrogen, wherein the electric resistivity changes repeatedly at a repeating cycle of from 1 nm to 3 nm or discontinuously in the direction of thickness so that the maximum value differs from the minimum value by at least two digits.
(4) J-P-A No. 1013/1999 describes a thermal head having, as a protective film, a laminated film of a carbon layer containing carbon as a main component, and a metal layer containing, as a main component, an alloy of semimetals or metals having at least one or more, or at least two or more selected from the group consisting of Si, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, wherein the film thickness of each layer is set to be from 0.5 nm to 50 nm, and the carbon layer/metal layer ratio is set at from 0.01 to 100.
(5) J-P-A No. 72288/1998 describes the following technique. A film-forming raw material gas containing a carbon compound gas is made into plasma through the application of voltage under vacuum conditions to produce a carbon film in which one or more than one carbon layer units each having a 10 nm to 200 nm-dia fine-grain thin-film carbon layer and a fine-grain carbon layer are formed. By the carbon film thus produced, stress is reduced, so that improvement of the adhesion, and improvement of the durability by an increase in film thickness are implemented.
(6) J-P-A No. 298097/1997 describes a laminated product having a DLC laminated film, wherein a conductive film, and a film having a higher hardness than that of the conductive film are alternately stacked into at least three or more layers so that the outermost layer is the conductive film.
The high frictional performance of a hard carbon film exemplified by DLC is considered to be attributable to the self lubricity resulting from sliding at the xcfx80 bond plane of the graphite crystal included therein, or formed at the sliding interface during sliding. Therefore, the graphite itself tends to be deformed, and hence for such a hard carbon film containing fine-grain graphite in itself, the friction coefficient is low and the frictional performance is good, while the film hardness itself is low, so that wear due to sliding is severe. Namely, if a low friction coefficient is sought, a high hardness cannot be obtained, or if the hardness is low even with a low friction coefficient, the wear resistance is insufficient. In either case, there occurs the problem that the film has insufficient durability as a hard coating film. In contrast, when the content of the graphite component in the DLC film is set to be low, the film hardness can be increased to ensure the wear resistance. However, reduction of the friction coefficient due to self lubricity of graphite is not attained sufficiently. Therefore, even among conventional DLCs of single-layer structure and multilayer structure, a DLC film achieving a friction coefficient as low as 0.15 or less, i.e., at least practically required value, and about 0.1 with stability, and having high wear resistance has not been implemented.
Further, for metal nitrides such as TiN, TiAlN, and CrN conventionally used as hard coating film materials, the friction coefficient is reduced by utilizing splashed particles referred to as macroparticles arising during film formation, and wear particles arising from attack on the counterpart material by the friction with the counterpart material, or attack by the counterpart material as lubricating materials. However, use of such a hard material inevitably entails troubles such as wear of a sliding component, an increase in friction coefficient with time, and clogging due to the wear particles.
The present invention has been achieved in view of the foregoing problems. It is therefore an object of the present invention to provide a coating film and a coated machine component that have an excellent wear resistance, a low friction coefficient, and an excellent sliding performance.
A coating film in accordance with a first aspect of the present invention is a DLC hard multilayer film, which has a low hardness hard carbon layer containing a graphite cluster having a mean size of 2 nm or more, and a high hardness hard carbon layer containing a graphite cluster having a mean size of 1 nm or less, the low hardness hard carbon layer and the high hardness hard carbon layer being amorphous structures each containing carbon as a main component, and being alternately laminated.
In accordance with a second aspect of the present invention, it is preferable that the graphite cluster in the low hardness hard carbon layer has a shell-like structure in which the C-planes (xcfx80 bond planes) of graphite are laminated in the form of spherical nuclei. The form of spherical nuclei can also be expressed as the form of concentric spheres or the onion-like form. It can be supposed that the cluster having such a shell structure assumes a structure similar to that of a carbon onion such as C1500 in which fullerene clusters form a nested structure. Further, in accordance with a third aspect of the present invention, it is preferable that the layer thickness of the low hardness hard carbon layer is from not less than 2 nm to not more than 30 nm, and the layer thickness of the high hardness hard carbon layer is from not less than 0.4 nm to not more than 10 nm. Still further, in accordance with a fourth aspect of the present invention, it is preferable that the ratio T1/T2 of the layer thickness T1 of the low hardness hard carbon layer to the layer thickness T2 of the high hardness hard carbon layer is from 5 to 0.2. Furthermore, in accordance with a fifth aspect of the present invention, it is preferable that the outermost layer is formed of the low hardness hard carbon layer, and the layer thickness thereof is from 2 nm to 200 nm. In this case, it is preferable that the thickness of the laminated portion inner than the outermost layer is set larger than the layer thickness of the outermost layer by at least 500 nm or more.
A component excellent in wear resistance and sliding performance in accordance with a seventh aspect of the present invention is formed by laminating the DLC multilayer film in accordance with any one of the first to sixth aspects on a substrate with an adhesion-improving intermediate layer interposed therebetween.